1. Field of the Invention
The present invention relates to a surface acoustic wave sensor arranged to detect a target substance on the basis of a change in frequency due to mass load. More particularly, the present invention relates to a surface acoustic wave sensor including a reaction film arranged to react with a target substance to be detected or a binding substance that binds to a target substance to be detected.
2. Description of the Related Art
To date, various sensors using surface acoustic wave devices have been proposed. For example, WO2005/003752 discloses a surface acoustic wave sensor having a structure, a cross-sectional front view of which is schematically shown in FIG. 12.
In a surface acoustic wave sensor 501, an IDT electrode 503 is disposed on a piezoelectric substrate 502 in order to excite surface acoustic waves. The IDT electrode 503 includes a main electrode layer 503a composed of Au or the like, and adhesion layers 503b and 503c composed of Ti or the like disposed on the upper surface and the lower surface of the main electrode layer 503a. A protective film 504 composed of SiO2 is disposed so as to cover the IDT electrode 503.
A reaction film 505 mainly composed of an organic material, such as a synthetic resin, is disposed on the protective film 504. The reaction film 505 is composed of a resin composition prepared by mixing a reactant which reacts with a target substance to be detected into a synthetic resin.
When a liquid containing a target substance to be detected or the like is brought into contact with the reaction film 505, the target substance to be detected binds to the reaction film 505. As a result, the mass load on the IDT electrode 503 increases. Due to the change in the mass load, the frequency of a surface acoustic wave excited by the IDT electrode 503 changes. The target substance to be detected can be detected on the basis of the change in frequency.
The adhesion layer 503b is provided in order to enhance the adhesion of the IDT electrode 503 to the piezoelectric substrate 502, and the adhesion layer 503c is provided in order to enhance the adhesion of the IDT electrode 503 to the protective film 504. The protective film 504 is provided in order to protect the IDT electrode 503.
In the surface acoustic wave sensor 501 described in WO2005/003752, a target substance to be detected is bound to the reaction film 505. However, the reaction film 505 is composed of an organic material, and therefore, when a liquid containing the target substance to be detected comes into contact with the surface of the reaction film 505, the liquid component, e.g., water, tends to permeate through the reaction film 505, be absorbed by the protective film 504, and reach the IDT electrode 503. Furthermore, the adhesion layer 503c composed of Ti or the like may absorb moisture, and as a result, the frequency of the surface acoustic wave excited tends to vary. That is, the drift of the frequency characteristic increases, and it becomes difficult to detect with high accuracy a change in frequency due to a small change in the mass load.